Highlights

Not all rooftop solar PV plants generate power during power failure; only some do

Whether the plant generates electricity during power failure or not lies with the inverter

The inverter matches the power from the solar plant with another source of stable power to ensure quality of electricity supplied. If another source is not available the inverter will not deliver power

The inverter can also shut down the solar plant in the event of grid failure for the safety of those repairing the grid

Only grid-interactive or hybrid inverters (which are a kind of grid-interactive inverter) will provide electricity even during power failure because they can utilise several sources of power, not just grid power, to provide the reference voltage that solar PV should match

All solar PV power plants that deliver AC power require a reference voltage, whether from grid power or battery or diesel genset, to function

Do all rooftop solar plants generate power during power failure?

Not all rooftop solar plants generate power during a power failure. This comes as a surprise to many of our clients, but solar plants have different designs for different purposes, and some designs benefit from the plant not generating power during a power failure.

Why some rooftop solar plants shut down during power failure

There are two important reasons why some solar plants shut down during a power failure

Reference Voltage

As the amount of sunlight falling on the panels varies during the day (due to clouds, etc.), the power output from the panels also varies. As this variation could damage equipment that is powered by solar, the inverter continuously matches the PV plant’s output to another source of steady power. Therefore a rooftop solar PV that generates AC power will always needs another source of power (whether the grid or diesel generator or batteries) to provide a reference voltage in order to function. If the inverter is designed to use only grid power as a reference voltage, the plant will not generate power even if there is ample sunlight.

Anti-islanding

When a power failure occurs, a portion of the grid isn’t energised. This non-energised portion of the grid is known as an island. If the solar plant is pumping electricity into this non-energised portion of the grid during a power failure, it might cause utility personnel who are working on the grid to be electrocuted. To eliminate this risk, the inverter in the solar power system turns off the power from the plant.

Kinds of inverters

As it is the inverter that determines whether the plant continues to function or not during a power cut, we need to only understand the different kinds of inverters to ensure we have a rooftop solar plant that generates electricity even during power cuts. There are 4 kinds of inverters

Grid-tied –These inverters are primarily designed to supply the generated power to the grid and also power the load while grid power is available. This inverter will NOT generate power during a power failure because it uses only grid power as a reference voltage and cannot function in the absence of grid power

Off-grid – These inverters do not work with grid power and are designed to work only with a battery backup or diesel generator in off-grid applications. They are suitable for applications where grid power is not available at all, but are not the right choice if you need your solar plant to work in conjunction with grid supply

Grid-interactive –These inverters work both with the grid supply and with either a battery backup or diesel generator to support the load even during a power failure.

Hybrid inverters (also known as bidirectional or magical inverters) are a one system solution for a complete solar PV system. They can automatically manage between 2 or more different
sources of power (grid, diesel, solar). They have inbuilt charge controllers, MPPT controller, Anti Islanding solutions, DC and AC disconnects and other features like automatic turning on/off of the diesel generator, automatic data logging, and various kinds of protection for the different components of the system, making them ideally suited for applications that require management of power from different sources

It is therefore critical to understand the purpose the rooftop solar PV plant is to fulfil before selecting the inverter. As vendors use various terms to refer to different components we urge you to verify if the inverter will supply power during power failure by specifically discussing this issue with the vendor, rather than going by any label assigned to the product.

Takeaways

Not all rooftop solar PV plants generate power during power failure; only some do

Whether the plant generates electricity during power failure or not lies with the inverter

The inverter matches the power from the solar plant with another source of stable power to ensure quality of electricity supplied. If another source is not available the inverter will not deliver power

The inverter can also shut down the solar plant in the event of grid failure for the safety of those repairing the grid

Only grid-interactive or hybrid inverters (which are a kind of grid-interactive inverter) will provide electricity even during power failure because they can utilise several sources of power, not just grid power, to provide the reference voltage that solar PV should match

All solar PV power plants that deliver AC power require a reference voltage, whether from grid power or battery or diesel genset, to function